This invention relates to a system and procedure for inspection of a surface of a workpiece and, more particularly, to the inspection of the surface of a semiconductor wafer for contamination by particles which may develop in the manufacture of integrated electric circuits.
Integrated circuits are manufactured on relatively large wafers which are cut by a saw to separate the individual circuit chips. Portions of the surface of the wafer may be built up with layers of material and may also be etched, particularly at the sites of circuit chips, to impart a pattern to the surface, which pattern is characterized by undulations in the wafer surface. Other portions of the wafer surface, particularly regions between the circuit chips reserved for the kerf of the saw, the saw-cut regions being the kerf, may be bare of an undulation pattern and have the characteristics of a specularly smooth surface.
During the process of manufacturing the wafers, it is a common occurrence for particles of various materials employed in the manufacturing process to come to rest upon the wafer surface. Such particles alight on both the smooth and the patterned portions of the surface of a wafer. Such particles act as a contaminant, and would interfere with the proper operation of the electric circuits if allowed to remain on the surface. Accordingly, one step in the manufacturing process is the cleaning of the wafer surfaces to remove particulate contamination. Thereafter, it is necessary to inspect the wafer surfaces to insure that they have been cleaned adequately of the contamination.
Of particular interest herein is inspection of a wafer surface by optical inspection equipment. The operation of such equipment is based on the observation that a reflection of illuminating light from a wafer surface depends on the direction of illumination relative to the wafer surface, and also on physical characteristics of the surface. Specific physical characteristics of the surface affecting reflection of the illuminating light are smooth regions, undulating regions, and particles. The smooth regions can produce high intensity reflections in a specific direction. The undulating regions can produce both strong reflected signals at specific angles as well as intense scatter in many directions. A particle induces a reflection which is relatively weak and is scattered in many directions.
A problem arises in that equipment which has been configured to process the weak scattered reflected light from a particle can be rendered inoperative by exposure to the intense scatter from a smooth or patterned region. In the case of a smooth surface region, free of undulations and illuminated by a beam of light normal to the surface, a path of reflected light can be predicted because it is known that the reflected light will be also normal to the surface. However, the problem of dealing with reflected light is compounded in the case of the undulations of a patterned region because the surface thereof can reflect light in any one of many possible directions depending on the local orientation of a part of the surface receiving an incident beam of illuminating light.
To avoid excessive reflected light in the use of optical inspection equipment, one form of equipment illuminates the wafer surface normally, and views reflected light at a glancing angle by light detectors positioned at or near the surface. Another form of equipment provides for a viewing of a surface defect along a normal to the surface in response to both horizontal and vertical illumination of the subject.